Currently, there are two main trends in the design and manufacture of compressors for use in vehicle air-conditioning systems. These are the use of the more environmentally friendly carbon dioxide (CO2) as a refrigerant to replace tetrafluoroethane (R134a) and the refrigerant for smaller, lightweight compressors which take up as little space and have as low a weight as possible. The latter trend arises as a result of the desire to produce smaller and lighter vehicles which are more fuel efficient. It is also a requirement that the compressor itself be energetically efficient.
In essence, the two trends conflict because the use of CO2 as the refrigerant in the compressor requires it to operate at a higher pressure than a conventional system using R134a as a refrigerant and this leads to the requirement for the compressor to be made from high pressure components, such as steel, which is heavier than materials such as aluminum that can be used to manufacture compressors for use at lower pressures. If aluminum is used for the casing of a high pressure compressor then the wall thickness of the casing must be increased. Also, in high pressure compressors the casing must be adequately sealed.
DE 19621174 describes a compressor suitable for use with CO2 as the refrigerant in which a casing defining a cylinder block is sealed on its drive shaft side by an end member which is screwed to the casing by a large number of small diameter bolts. By using smaller bolts, the diameter of the screw holes in the wall of the casing can be kept small and the wall of the casing can therefore be made thin. However, the design of the casing is such that the driving mechanism of the compressor and the pistons especially are difficult to mount. Also, despite the compact design of the compressor and the thin wall of the casing, the casing and the pistons were made of steel.
DE 19833604 similarly describes a compressor for use with CO2 as the refrigerant which is also made of high-strength materials such as high pressure steel, bronze alloy and the like. To keep the weight of the compressor to a minimum, its casing has a low wall thickness but this does not allow screws or bolts to be secured therein. Accordingly, in this compressor, the bolting arrangements of the casing to an end member is made by passing bolts through casing and into the cylinder head. However, this requires mounting space to be provided. The cylinder head of a compressor must provide a certain volume in order that suction and pressure gas pulses are reduced. Also, various functions of the compressor, such as the regulation of the compressor and oil, separation take place in the cylinder head. Consequently, the compressor in question tends to be bulky. The weight of the compressor is also added to by the fact that a considerable number of very lengthy bolts is required. A further drawback of the compressor is that each bolt must be individually sealed because its head projects out of the casing.
A similar arrangement is described in DE 19947347.
It will be appreciated with such compressors that if, to reduce weight, the casing is made from an aluminum problems will arise with regard to sealing when the compressor heats up and cools down because the length of the bolts, which must of necessity be made from steel, causes them to expand and to contract at a different rate to the casing. As sealing of particularly high pressures in the vicinity of the bold head is required, this problem can be severe.
A further problem arises in swash or wobble plate compressors relating to the requirement to prevent the plates from rotating during use. A conventional piston 1, as shown in FIG. 1, for use in such a compressor typically comprises a body 2 with a head portion 3 at one end for reciprocation in a bore and a foot portion 4 at its other end. A neck or bridge 5 links the foot portion 4 to the body 2 so that a recess 6 is defined between foot portion 4 and body 2. Recess 6 is intended to accommodate a bearing of a swash or wobble plate arrangement by which means the piston is reciprocated.
Conventionally, body 2 of the piston has a circular transverse cross-sectional profile, as does the bore in which it reciprocates. This necessitates the use of an anti-rotation lock to prevent any significant rotation of the piston about its longitudinal axis. Various mechanisms have been used to this end. For example, the body of the piston can be provided with a spine or ridge which projects longitudinally along its length and which reciprocates within a similarly extending and matching groove in the wall of the bore. In EP 0740076 it is proposed that the bridge of the piston be enlarged so that it defines a convex outer wall apposed to the concave wall portion of the casing next to which it reciprocates. The radius of the convex face of the bridge is made greater than the radius of the cylindrical body of the piston but smaller than the internal concave wall of the casing. Consequently, owing to contact between the enlarged bridge portion and the inner wall surface of the casing which occurs as a result of rotation of the piston during use, the actual degree of the rotation is limited. In fact, only an edge or spot of the convex face of the bridge of the piston contacts the wall of the casing. Also, the further this edge is from the longitudinal axis of the cylindrical body of the piston, the longer is the theoretical lever arm and therefore the lower is the bearing stress for supporting the piston rotation. This keeps the frictional forces caused by the contact between the piston and the casing wall low. However, it will be appreciated that the contacting surfaces should be treated to reduce friction and wear so far as is possible.
In U.S. Pat. No. 6,325,599 an anti-rotation piston for a swash plate compressor is described that includes a pair of opposed anti-rotation wings that extend radially from one end of the body adjacent the bridge. The wings prevent rotation of the piston as they contact the wall of the casing. However, this arrangement has the significant disadvantage that the guide length of the piston body is reduced.